Re: probably cause (and fix) for floating-point assist faults on itanium

Claudio Freire <klaussfreire(at)gmail(dot)com> writes:
> On Thu, Nov 17, 2011 at 10:07 PM, Greg Matthews
> <gregory(dot)a(dot)matthews(at)nasa(dot)gov> wrote:
>> if (smoothed_alloc <= (float) recent_alloc)
>> smoothed_alloc = recent_alloc;
>> else if (smoothed_alloc >= 0.00001)
>> smoothed_alloc += ((float) recent_alloc - smoothed_alloc) /
>> smoothing_samples;
>>

> I don't think that logic is sound.

> Rather,

> if (smoothed_alloc <= (float) recent_alloc) {
> smoothed_alloc = recent_alloc;
> } else {
> if (smoothed_alloc < 0.000001)
> smoothed_alloc = 0;
> smoothed_alloc += ((float) recent_alloc - smoothed_alloc) /
> smoothing_samples;
> }

The real problem with either of these is the cutoff number is totally
arbitrary. I'm thinking of something like this:

/*
* Track a moving average of recent buffer allocations. Here, rather than
* a true average we want a fast-attack, slow-decline behavior: we
* immediately follow any increase.
*/
if (smoothed_alloc <= (float) recent_alloc)
smoothed_alloc = recent_alloc;
else
smoothed_alloc += ((float) recent_alloc - smoothed_alloc) /
smoothing_samples;

/* Scale the estimate by a GUC to allow more aggressive tuning. */
upcoming_alloc_est = smoothed_alloc * bgwriter_lru_multiplier;

+ /*
+ * If recent_alloc remains at zero for many cycles,
+ * smoothed_alloc will eventually underflow to zero, and the
+ * underflows produce annoying kernel warnings on some platforms.
+ * Once upcoming_alloc_est has gone to zero, there's no point in
+ * tracking smaller and smaller values of smoothed_alloc, so just
+ * reset it to exactly zero to avoid this syndrome.
+ */
+ if (upcoming_alloc_est == 0)
+ smoothed_alloc = 0;

/*
* Even in cases where there's been little or no buffer allocation
* activity, we want to make a small amount of progress through the buffer

regards, tom lane